Goodies option:

Press Triangle, Square, Circle, Left, Square, Triangle, Square, Circle, Left, Square within three seconds at the "Press Start" screen. An additional selection will appear at the end of the options menu that allows level selection, all weapons, unlimited ammunition, and the ability to view the FMV sequences.

PS1 | Submitted by GamesRadar

Turbo mode:

Press X, Square, Triangle, Circle, X, Square, Triangle, Circle, L2, R2 within three seconds at the "Press Start" screen. A white screen flash, sound effect, and display of the cheat name will confirm correct code entry. Press Circle during game play to run faster. Note: This will also use any item currently being held by your character.

PS1 | Submitted by GamesRadar

Nakomi mode:

Press Up(2), Down(2), Left, Right, Left, Right, X, Circle within three seconds at the "Press Start" screen. A white screen flash, sound effect, and display of the cheat name will confirm correct code entry. All 3D characters except the Boss will be flattened.

PS1 | Submitted by GamesRadar

Sepia graphics:

Press Left(10) within three seconds at the "Press Start" screen. A white screen flash, sound effect, and display of the cheat name will confirm correct code entry.

PS1 | Submitted by GamesRadar

Grayscale graphics:

Press X(2), Triangle(2), Square(2), Circle(2), Up, Down within three seconds at the "Press Start" screen. A white screen flash, sound effect, and display of the cheat name will confirm correct code entry.

PS1 | Submitted by GamesRadar

Big Head aliens:

Press Square, Circle(2), Square, Up, Square, Circle(2), Square, X within three seconds at the "Press Start" screen. A white screen flash, sound effect, and display of the cheat name will confirm correct code entry. Note: This code does not affect the Boss.

PS1 | Submitted by GamesRadar

Stage select:

Enable the "Goodies option" code(see below). Then, press Left, Right, X, Triangle, Left, Up within three seconds at the "Press Start" screen. A white screen flash, sound effect, and display of the cheat name will confirm correct code entry. An option that will allow selection of an individual stage within a level will become available. Note: This code may cause music and graphic problems.